In recent years, the use of a variety of gases within the production processes for products has become common, and in production plants for semiconductors, because gas-based chemical reaction processes are employed on single crystal silicon substrates, volatile or toxic gases are widely used. Hydrogen gas is used in large quantities as the carrier gas for these gases. However, hydrogen gas is itself highly explosive, and any leaks of hydrogen gas must be detected immediately.
In addition, there is now much concern about the depletion of fossil fuels such as petroleum, and many different options are being researched as potential replacement energy sources. Hydrogen can be readily obtained by the electrolysis of water, and generates water as a combustion product, with no emission of CO2, NOx or SOx or the like, and can consequently be claimed to be a very superior next-generation energy source.
In terms of techniques for converting hydrogen energy into electrical power, fuel cells that use the chemical reaction between hydrogen and oxygen are attracting the most attention. In particular, fuel cell vehicles fitted with fuel cells are being viewed with considerable hope as likely “favorites for environmentally friendly vehicles”. However, because hydrogen is the lightest and smallest molecule it is prone to leakage, and because it also ignites readily and combusts rapidly, it is an extremely dangerous gas. As a result, if a hydrogen energy system develops in the future, then the positioning of hydrogen gas sensors can be expected to become increasingly important.
Currently, semiconductor-based sensors that use a metal oxide are the most representative hydrogen gas sensors. Although these sensors exhibit high sensitivity and a high level of reliability, the sensor element itself must be heated to a high temperature. As a result, there are limits on the levels of miniaturization, weight reduction, power consumption reduction, or cost reductions that can be achieved for this type of sensor, and it is thought that these sensors will be unsuitable for a large variety of applications.
A specific example of a hydrogen gas sensor is that disclosed in Japanese Patent Laid-Open No. Sho 59-120945. This publication proposes a hydrogen gas sensor comprising a pair of opposing electrodes formed on one surface of an insulating substrate, a gas-sensitive film (SnO2) that covers these electrodes, a heater fitted to the opposite surface of the substrate, lead wires connected to this heater, and a catalyst layer (such as Pt) formed on top of the gas-sensitive film. However, in this hydrogen gas sensor, because the catalyst layer is formed by screen printing, controlling the film thickness is difficult, leading to large fluctuations in the film thickness, and making control of the properties of the sensor difficult. In addition, this hydrogen gas sensor also suffers from the drawback of having a high operating temperature of approximately 400° C.
Furthermore, in a semiconductor production plant, for example, isopropyl alcohol is used as a cleaning agent, and is always present in gaseous form in the air. Under these types of conditions, reliably detecting hydrogen gas leakage can be difficult, and as a result, providing a non-gas-sensitive thin film layer (an oxide such as SiO2 or alumina or the like) on top of the gas-sensitive film has been proposed, as in Japanese Patent Laid-Open No. Hei 01-250851, but the process for producing this non-gas-sensitive thin film layer is difficult, meaning cost increases are unavoidable, and the control costs associated with controlling the properties of the sensor also increase.
In addition, there have been reports of gas detection elements that use a vapor deposition film of phthalocyanine, but these elements are used for monitoring the level of electrical conductivity accompanying gas adsorption and desorption, exhibit no selectivity between electron-donating and electron-withdrawing gases, and suffer from extremely unstable operation.